1. Field of the Invention
The present invention relates generally to heterojunction semiconductor devices. More particularly, the invention relates to a heterojunction bipolar transistor having a raised or improved breakdown (withstand) voltage, and a semiconductor integrated circuit device using the same.
2. Description of the Related Art
To improve the collector-to-emitter breakdown voltage of heterojunction bipolar transistors during operation, it is important to suppress or prevent avalanche breakdown in the collector region. To realize this, an improved structure has been developed and disclosed, where a semiconductor layer having a low or small impact ionization coefficient is inserted into a high electric-field part of the collector region. This structure is disclosed in, for example, the Japanese Non-Examined Patent Publication No. 7-16172 published in 1995.
FIG. 1 shows schematically the energy band diagram of a prior-art heterojunction bipolar transistor where a semiconductor layer having a low or small impact ionization coefficient is not inserted into the collector layer region. This transistor comprises an emitter layer 105, a base layer 106, a collector layer 113, and a sub-collector layer 104.
When no or a low collector current flows, the bottom EC of the conduction band and the top EV of the valence band in the collector layer 113 are given by the chain lines B1 and B2 in FIG. 1, respectively. When a high collector current flows, the space charge increases and therefore, the bottom EC of the conduction band and the top EV of the valence band in the collector layer 113 are respectively raised, as shown by the solid lines A1 and A2 in FIG. 1. As a result, as the collector current increases, the electric-field strength on the base side of the collector layer 113 tends to decrease and at the same time, the electric-field strength on the sub-collector side of the collector layer 113 tends to increase.
FIG. 2 shows schematically the energy band diagram of an improved prior-art heterojunction bipolar transistor to improve the breakdown voltage. This transistor has the same structure as shown in FIG. 1 except that a semiconductor layer 114 is inserted into the collector layer 113. The layer 114 has a wider energy band gap than the layer 113. This structure is disclosed in, for example, the above-identified Publication No. 7-16172.
As seen from the energy band structure of FIG. 2, the semiconductor layer 114 is additionally provided on the sub-collector side of the collector layer 113, which is a high electric-field part. The wider the band gap, the lower the impact ionization coefficient and the higher the breakdown voltage. Therefore, avalanche breakdown is more difficult to occur in the semiconductor layer 114 than in the collector layer 113. This means that the breakdown voltage during operation is raised or improved.
With the energy band structure of FIG. 2, dependent on the type or sort of semiconductor materials used, there arises a possibility that “conduction band discontinuity” occurs between the collector layer 113 and the semiconductor layer 114 which are different in band gap from each other. If so, the high-frequency characteristics of the transistor will deteriorate because of the accumulation and retention effect of carriers caused by the conduction band discontinuity. To suppress the deterioration of the high-frequency characteristics, it is effective to introduce a p-n junction with high doping concentrations between the layers 113 and 114. This is disclosed in the Japanese Non-Examined Patent Publication No. 7-193084 published in 1995. The energy band structure disclosed in this Publication is shown in FIG. 3.
As shown in FIG. 3, a p+-type In0.53Ga0.47As layer 116a doped heavily with a p-type dopant and a n+-type InP layer 116b doped heavily with a n-type dopant are introduced to form a p-n junction between an i-type In0.53Ga0.47As collector layer 113a and a n-type InP collector layer 115. The reference numerals 105a, 106a, and 104a are an emitter layer, a base layer, and a sub-collector layer, respectively.
FIG. 4 shows the energy band structure of a prior-art heterojunction bipolar transistor, which improves both the breakdown voltage and the high-frequency characteristics. This is disclosed in the Japanese Non-Examined Patent Publication No. 6-326120 published in 1996. This structure is obtained by inserting a heavily doped p-type GaAs layer 116 between the i-type GaAs collector layer 113 and the n-type AlGaAs layer 114 having the wider band gap than the layer 113 in the structure of FIG. 2. Due to the insertion of the GaAs layer 116, the electric-field in the collector layer 113 is relaxed and therefore, the electrons are restrained from entering their high-energy states to thereby suppress the reduction of their velocity. Thus, not only the breakdown voltage is raised but also the high-frequency characteristics are improved.
However, the above-described prior-art energy band structures shown in FIGS. 2 to 4 have the following problems.
Specifically, with the prior-art structure of FIG. 2, no measure is taken to restrain the high electric-field part in the collector layer 113 from expanding toward the base layer 106 with the increasing collector current. Therefore, as the collector current increases, the high electric-field part will expand toward not only the semiconductor layer 114 but also the collector layer 113. As a result, a problem that the breakdown voltage decreases arises.
With the prior-art structure of FIG. 3, the p+-type In0.53Ga0.47As layer 116a and the n+-type InP layer 116b relax the band gap discontinuity apparently. However, these layers 116a and 116b form a high doping-concentration p-n junction and thus, very high electric field is generated in the layer 116a. As a result, there arises a problem that avalanche breakdown is likely to occur in the layer 116a. This means that the breakdown voltage is likely to lower. Moreover, even if the band gap discontinuity is apparently relaxed, this is unable to be really eliminated.
In addition, if the collector layer 113 and the semiconductor layer 114 in FIG. 2 are respectively made of GaAs and InGaP, the following problem will occur.
If the InGaP layer 114 is grown to form its natural superlattice, almost all the band gap discontinuity at the interface of the GaAs layer 113 and the InGaP layer 114 can be eliminated. Thus, the energy band structure of FIG. 3 is unnecessary. This means that the energy band structure of FIG. 2 simpler than that of FIG. 3 can be used. However, if so, there arises a problem of degradation of the rising characteristic of the collector current-collector voltage characteristic.
With the prior-art structure of FIG. 4, the p+-type GaAs layer 116 is a heavily doped p-type semiconductor and thus, the potential in the layer 116 is raised. If the n-type AlGaAs layer 115b is replaced with a n-type InGaP layer, the potential of the layer 116 is raised furthermore. As a result, there arises a problem of degradation of the rising characteristic of the collector current-collector voltage characteristic, as well.